The present invention relates to heat recovery steam generators and, more particularly, to economizer sections of heat recovery steam generators.
Heat recovery steam generators are employed to extract sensible heat remaining in the exhaust of a gas turbine to produce an overall system which better utilizes the thermal energy available in the fuel. A typical gas turbine, operating independently, exhibits a thermodynamic efficiency of on the order of 30 percent due to the large amount of heat discharged in its exhaust. The addition of a heat recovery steam generator to harvest the exhaust heat can result in a plant system having an overall thermodynamic efficiency exceeding 70 percent. This increase in efficiency is attained at the price of additional capital investment. Increasing fuel costs are providing economic incentives for taking advantage of the increases in thermodynamic efficiency made available by adding a heat recovery steam generator to a gas turbine.
A heat recovery steam generator typically consists of an evaporator containing an array of tubes past which the gas turbine exhaust gasses are flowed. Water in the tubes is evaporated to produce saturated steam. The saturated steam is circulated through coils of a superheater which are interposed in the exhaust gas flow upstream of the evaporator to receive the hottest gas. The saturated steam is superheated in the superheater before being delivered to a using process such as, for example, a steam turbine. Downstream of the evaporator, the gasses may be flowed past the tubes of an economizer to heat feedwater flowing therethrough. Finally, the gasses leaving the economizer may be fed through a recuperator to heat incoming combustion air and/or through an evaporator of a low-pressure steam generator. Additional elements, not of concern to the present disclosure, may be included such as, for example, auxiliary burners, auxiliary bypass stacks and feedwater de-aerators. The problem which the present invention seeks to solve is flow instability and possible early tube failure due to the unwanted generation of steam in the tubes of the economizer.
Two general types of heat recovery steam generators are differentiated by the manner in which water and steam circulation is achieved in the evaporator. The two types are known as forced circulation and natural circulation heat recovery steam generators.
In a forced circulation heat recovery steam generator, one or more pumps are employed to force water and steam through generally horizontal tubes in the evaporator. The steam and water mixture, is circulated through a steam drum in which the steam and water are separated. The steam is separated from the mixture and sent to the superheater; the water is returned to the evaporator. The configuration of the economizer in a forced circulation heat recovery steam generator is dictated by the configuration of the evaporator and its direction of gas flow. That is, the tubes in the economizer of such a system are conventionally disposed horizontally.
A natural circulation evaporator, in contrast, generally employs vertically oriented tubes past which the exhaust gasses are flowed in the horizontal direction. A steam and water mixture from the evaporator tubes flows upward to a steam drum located above the evaporator tube bank. The steam is separated from the water in the steam drum and is sent to the superheater or using process. The water from the steam drum is returned to the bottom of the evaporator tube bank through downcomer tubes or pipes. The difference between the mean mixture density in the evaporator riser tubes and the mean water density in the downcomer tubes applied over the height of the evaporator tube bank establishes a net gravity head which drives a natural circulation in the evaporator tube bank without the need for pumping to maintain such circulation. Efficient interfacing with constant gas path dimensions generally leads to an economizer which also utilizes vertically oriented tubes.
An economizer with vertical tubes is conventionally fabricated in a plurality of tube planes. Each tube plane includes a parallel array of tubes connected at top and bottom to headers. A tube plane may consist of one or more rows per pass. For example, feedwater is pumped into an upper inlet header and flows downward in parallel through the tubes of the first pass to a second lower header. It then flows upward to the next header and continues in such a serpentine pattern toward the final outlet header. The feedwater gains sensible heat released by the hotter exhaust gas by forced convection heat transfer through finned tubes during each pass until the heated feedwater finally reaches the outlet from which it is connected to the steam drum.
When the feedwater flows upward through an economizer tube plane the buoyancy developed as the feedwater gains heat aids the pumping pressure head between the top and bottom headers of the tube plane. Both pumping pressure head and buoyancy thus aid circulation in upward flowing tube planes. In tube planes in which the feedwater flows downward, the buoyancy no longer aids the pumping pressure head, but instead, the buoyancy opposes the pumping pressure head. Since the tube planes in the economizer are in series, the total end-to-end feedwater flow is determined by the amount of feedwater passing the boiler feed pump which is, in turn, controlled by the amount of steam being demanded by the using process. This does not necessarily imply, however, that all tubes in a tube plane carry the same flow. Since all tubes within a particular tube plane are in parallel, it is possible for one or more tubes to carry substantially less feedwater with the remainder of the tubes carrying slightly more.
At low feedwater flow rates such as encountered, for example, during startup, the total feedwater flow may be so low that flow stagnation reversal and/or steaming may occur in one or more tubes of a tube plane. If steaming should occur in a tube plane having upward flow, the steam and water readily flow upward to maintain a net positive (forward) flow in all of the tubes in the tube plane. The same is not true of the downflowing tube planes. Since the pumping pressure head and the buoyancy compete with each other in tube planes having downward flowing water, the problem of unsteady flow and potential steaming is accentuated in those locations. The buoyancy in one or more tubes in a downflowing tube plane may reduce the flow rate therein sufficiently to increase the residence time of the feedwater to a value which permits steaming to begin. The presence of steam may partially or totally block further flow through the affected tubes or may, in fact, produce reverse flow therein. The net plenum-to-plenum flow remains positive due to increased flow through the remaining tubes of the tube plane.
Even without steaming, a sufficiently low driving pressure head at light loads may result in a multivalued hydraulic characteristic for the tube plane, where reversal to upward flow in one or more tubes of the plane is possible. If the flow reverses direction, sensible heat is still added, and steaming is a likely end result.
If an unsteady steaming condition in one or more tubes in a tube plane remains in existence for a period of time, the affected tube dries out, perhaps periodically, and becomes subject to thermal fatigue.
It is generally known that unstable flow in downflowing economizer tube planes due to steaming in one or more tubes can be a stable condition which, once begun, is able to maintain itself even when feedwater flow is increased well beyond a threshold value which is originally effective to prevent the onset of such unsteady flow. This results from the fact that, once flow stoppage or reverse flow occurs in a downflowing tube, a greater plenum-to-plenum pressure head is required to overcome it than is required to avoid reversal and/or steaming before it occurs.
Some of the known possible solutions to this problem are undesirable because of their negative effect on efficiency. One possible solution, of course, is the elimination of downflowing tube planes. This may be accomplished, for example, by returning the feedwater from the top plenums to the bottom plenums through return lines which are either outside the gas flow or are insulated from it. In this manner, all heating takes place in upward-flowing tubes. This solution requires substantially more tubes to achieve a desired amount of active heat transfer surface and, since it involves conveying the downflowing feedwater outside and then back into the gas path on each pass, it creates a substantial complexity in piping as well as an increase in cost.
Another possible solution includes employing tube inlet orificing to effectively increase the driving pressure head at a given mass velocity. This artificially increases the plenum-to-plenum pumping head across all of the tubes in a tube plane and thereby reduces the tendency for one or more of the tubes to generate steam. The required orifice size could be so small, however, that orifice plugging may be a concern. In addition, the flow restriction imposed by the orifices becomes a problem at high flow rates requiring increased pumping power to overcome it resulting in a potentially substantial performance penalty at high loading conditions during which performance is usually guaranteed. The use of orifices to solve a differential-flow problem in the heat exchanger tube bundle of a moisture separator reheater is disclosed in U.S. Pat. No. 4,206,802, the disclosure of which is herein incorporated by reference.
A further possible solution includes venting the economizer discharge headers of tube planes in which flow reversal or steaming may be a problem to the steam drum. This artificially reduces the discharge header pressure and thus increases the effective pumping head seen by the tube plane. This solution invokes a thermodynamic efficiency penalty as well as increased cost due to piping complexity and additional header and steam drum penetrations. The penalty in efficiency may be reduced or eliminated at high loading conditions above the stability threshold by shutting off the venting circuit, but a somewhat sophisticated control system would be involved.
Some forced circulation systems employ recirculation of heated water to discourage steaming and to reduce cold-end corrosion of the tubes. Such recirculation is driven in response to temperature and/or pressure in the system. None of these known forced-circulation systems, usually employing horizontal tubes, encounter the problems of economizer flow instability and reverse flow which the present invention addresses and solves.